XVI International Botanical Congess

Roots of intact Phragmites plantlets were secured horizontally in agar under 02-free N2, the leafy shoot was fully-exposed to air. Radial oxygen profiles through rhizosphere and root at different distances from the apex were obtained polarographically with servo-driver Clark-type beveled microelectrodes (tip diam. 5-20 um). The results confirm that the characteristic pattern of radial oxygen loss (ROL) along roots of wetland plants such as Phragmites viz. very high at the apex and sharply declining sub-apically, is caused by developing impedances in outer cell layers such as the exodermis. Cortical, oxygen, which increases from apex to base, was shown to rise most steeply in the apical parts where the cortex is non-aerenchymatous. The sub-apical decline in ROL was coincident with the appearance of aerenchyma but thin walled 'passage areas' in the exodermis through which lateral roots normally emerge remained leaky to oxygen.